1. Field of the Invention
The present invention relates to a motor drive circuit for controlling a sensorless motor.
2. Description of the Related Art
In electronic equipments using a disc media such as portable CD (Compact Disc) apparatus and DVD (Digital Versatile Disc), a brushless DC (direct current) motor for rotating the disc is used. The brushless DC motor generally includes a rotor with a permanent magnet, and a stator with coils of a plurality of phases that are star wire connected, where the coil is magnetized by controlling the electric current to be supplied to the coil, and the rotor is relatively rotated and driven with respect to the stator. The brushless DC motor generally includes a sensor such as Hall element and optical encoder to detect a rotation position of the rotor, where the current to be supplied to the coil of each phase is switched according to the position detected by the sensor, and an appropriate torque is applied to the rotor.
A sensorless motor for detecting the rotation position of the rotor without using a sensor such as Hall element is proposed to further miniaturize the motor (see e.g., patent documents 1, 2). The sensorless motor monitors a potential of the middle point wiring of the motor (hereinafter referred to as middle point voltage) and a back electromotive voltage (induced voltage) generated at one end of the coil, and obtains position information by detecting a zero crossing point in which the voltage becomes equal to the middle point voltage.
[Patent document 1] Japanese Patent Application (Laid-Open) No. H3-207250
[Patent document 2] Japanese Patent Application (Laid-Open) No. H10-243685
[Patent document 3] Japanese Patent Application (Laid-Open) No. H11-75388
FIGS. 1A to 1C are time charts showing a state of detecting the zero crossing point in pulse modulation drive. FIG. 1A shows a waveform chart of a pulse modulated signal PWM, FIG. 1B shows a waveform chart of a phase voltage (hereinafter also referred to as back electromotive voltage Vu) and a middle point voltage Vcom generated at the coil acting as a detection target of the zero crossing point, and FIG. 1C shows a waveform chart of a back electromotive detection signal BEMF_EDGE. In FIGS. 1A to 1C, the voltage to be applied to the coil of the motor is controlled through pulse modulation in an aim of continuously changing the current to be flowed to the coil of the motor in sinusoidal wave form or arch form.
As shown in FIG. 1B, noise component appears at a timing of transition from OFF to ON or at a timing of ON to OFF of the pulse modulated signal shown in FIG. 1A in the back electromotive voltage Vu generated at the coil that acts as a detection target of the zero crossing point. Due to such noise component, the back electromotive detection signal obtained by comparing the phase voltage Vu and the middle point voltage Vcom repeat high level and low level, and the zero crossing point is mistakenly detected. The mistaken detection of the zero crossing point is a mistaken detection of the position of the rotor, and causes problems such as degradation of rotation accuracy and rotation failure.